Most lymphoid malignancies arise by transformation of germinal center (GC) experienced B cells. In two prior funding periods we showed that the TCL1 proto-oncogene was abnormally expressed in samples from three major GC B cell lymphoma categories, including follicular (FL), Burkitt (BL), and diffuse large B cell (DLBCL) lymphomas. Two sets of Specific Aims addressed a causative role for aberrant TCL1 expression in the transformation of GC B cells and the mechanism(s) for regulating and dysregulating TCL1 expression in human B cell malignancies. Funding supported our group for 66 peer-reviewed publications. Now, we propose to logically expand the scope of prior successful work beyond TCL1 into an exciting new direction. During an immune response, B cells undergo rapid proliferation and remodeling of immunoglobulin (IG) genes within GCs to generate memory B and plasma cells. Unfortunately, DNA damage associated with this "GC reaction" also promotes most B cell malignancies. We recently discovered that ATM, activated by AID- dependent DNA double-stranded breaks (DSBs) during IG class switch recombination (CSR) in GC B cells, signals through LKB1 to inactivate CRTC2, a known transcriptional co-activator of CREB. Using genome-wide location analysis, we determined that CRTC2 inactivation unexpectedly repressed a genetic program that controls GC B cell proliferation, self-renewal, and differentiation into antibody (Ab)-secreting plasma cells while opposing lymphomagenesis (see Appendix- Sherman, et al., Molecular Cell, in press, 2010). Defects in this pathway were identified in pilot studies of human B cell lymphomas by ATM or LKB1 repression, or by a recently identified somatic mutation or genetic polymorphism in CRTC2. Much is known about CRTC2 as a regulator of glucose metabolism, and we have now shown that DSBs activate a pathway in GC B cells that inactivates CRTC2. However, no role for CRTC2 in cell differentiation or cancer has been described to date. In new preliminary studies, we discovered a set of CRTC2 bound genes from ChIP-chip in GC B cells that increase rather than decrease in expression with CRTC2 inactivation, suggesting that CRTC2 also has transcriptional repression activity beyond its CREB co-activator function. As a candidate regulator of cell differentiation and cancer, we propose Three New Specific Aims to investigate the role of CRTC2 in controlling B cell fate and function. In Aim 1, we will determine whether CRTC2 participates in transcriptional repression. In Aim 2, we will constitutively activate CRTC2 in GC B cells and evaluate effects on B cell differentiation and humoral immunity in vivo. In aim 3, we will determine whether a new activating CRTC2 alteration is a somatic mutation or germline polymorphism and we will investigate the necessity for CRTC2 inactivation to avoid lymphomagenesis. Overall, our studies expand the role for CRTC2 beyond metabolism and characterize an unexpected new regulator of B cell development and function.